The present invention relates to a security seal of the padlock type and, more particularly, to a security seal having a tampering indicator.
Security seals of the padlock type are generally well known, and have found wide spread use for sealing structures such as currency bags, closures, electric meters, and the like. Examples of such seals may be found in the following U.S. Pat. Nos. 3,375,033; 3,980,332; 4,353,583; 4,278,281; and 4,687,240.
All of these seals have, generally, a plastic body with a pair of spaced cavities in which a U-shaped shackle is permanently locked. Locking the shackle is typically accomplished by a pair of reversely bent, resilient legs that register with a bump or notch located in the spaced cavities. Although such devices have served the purpose, they have not proved entirely satisfactory, because some efforts to defeat such structures have proved successful.
In some case it is possible to remove the shackle portion by deforming the shackle and forcing it from the cavities, and thereafter reshape the shackle and reinsert it in the cavities without any apparent evidence that the seal had been opened. Prior art attempts to improve such seals have included various means to indicate tampering under such circumstances. For example, in U.S. Pat. No. 4,687,240 there is disclosed a seal having a pigmental polypropylene body which will produce a highly visible white blush when deformed by a shackle as it is forced from the seal body. The '240 seal also has a second tamper indicating feature. The exterior of the seal body has a slot into which the shackle ends will protrude if forced sufficiently to penetrate the relatively soft polypropylene seal body. Although such seals have served the purpose, they have not proved satisfactory under all circumstances.
It has been found that polypropylene is not stable when exposed to ultraviolet light for substantial periods. Therefore, after many years of use on electric meters exposed to sunlight, polypropylene seals have broken down, leaving the seals vulnerable to tampering. For example, the body of such seals have been known to loose their pliability, cease blushing when deformed, crack, etc., as a result of ultraviolet exposure for prolonged periods.
Therefore, those concerned with the development of such seals for long term use have turned to other materials that are stable in ultraviolet environments. One popular replacement material is acrylic, an inexpensive thermoplastic that is normally colorless. However, because acrylic does not blush, and is not sufficiently soft and pliable, many of the tamper indicating features of polypropylene seals are not possible in acrylic seals. Hence, those skilled in these arts have turned to other tamper indicating structures more suitable for seals made of materials such as acrylic. For example, the above-cited U.S. Pat. No. 3,980,332 shows a seal which includes a tampering indicator in the form of an insert that is used to lock the shackle, but the insert is made frangible and will normally break easily when attempts are made to separate the shackle from the seal body. Many seals of this type, however, may still be vulnerable to some forms of undetected tampering. Some seals with frangible inserts can be compromised by properly supporting the frangible insert while pulling on the shackle. As a result the deformed shackle can be removed from the body without breaking the frangible insert. Therefore, efforts to improve such seals have had only limited success.